Add config for ios-device-xcode-clang bot

ios/build/bots/chromium.mac/ios-device-xcode-clang.json

 {
   "owners": [
-    "dpranke",
+    "sdefresne",

[Dirk Pranke, 2016/11/16 01:09:40]

awww ...

[smut, 2016/11/16 01:12:44]

Maybe it should be you and him and not me. I don't know anything about the
differences between Chromium clang and Xcode clang. But this section in the
config has proven largely pointless.

     "smut"
   ],
   "comments": [
     "Builder for 32-bit devices.",
     "Build is performed with gn+ninja."
   ],
   "xcode version": "8.0",
   "GYP_DEFINES": [
     "OS=ios",
     "chromium_ios_signing=0",
     "component=static_library",
     "use_goma=1",
     "gomadir=$(goma_dir)",
     "target_subarch=arm32"
   ],
   "gn_args": [
     "additional_target_cpus=[ \"arm64\" ]",
     "is_debug=true",
     "target_os=\"ios\"",
     "ios_enable_code_signing=false",
     "is_component_build=false",
     "use_goma=true",
     "goma_dir=\"$(goma_dir)\"",
-    "target_cpu=\"arm\""
+    "target_cpu=\"arm\"",
+    "use_xcode_clang=true"
   ],
   "mb_type": "gn",
   "compiler": "ninja",
-  "additional_compile_targets": [ "gn_all" ],
   "configuration": "Release",
   "sdk": "iphoneos10.0",
   "tests": [
   ]
 }